Answer:
Priority: Counting number of ones (1s) in the unit of data, since 0 is zero.
Example: a unit of data: 1001 (=0x09) where number of 1s is 2 making even parity.
So, the parity bit - the most significant bit will be '0' - "even" which makes the byte 0000-1001 or 0x09.
Add 1, makes 10 - or xxxx-1010 still 2 bits set, making also even parity 0x0a.
Add 2, makes 11 - or 1000-1011 - 0x8b
You can make a simple transistor as a "gate" we call XOR. If one OR the other pin is raised, then raise pin out. If none or both are set, pin out is set low. The difference between OR and XOR is the result of both.
But use the "overflow" 4th pin of an OR "gate", and use this as input to the next bit and you "ADD" binary.